docs: def fractions_discrete(self, ds, n=None):
          r'''Computes the fractions of the cumulative distribution functions 
          which lie between the specified specified particle diameters. The first
          diameter contains the cdf from 0 to it. 
          
          Parameters
          ----------
          ds : list[float]
              Particle size diameters, [m]
          n : int, optional
              None (for the `order` specified when the distribution was created),
              0 (number), 1 (length), 2 (area), 3 (volume/mass),
              or any integer, [-]
  
          Returns
          -------
          fractions : float
              The differences in the cumulative distribution functions at the 
              specified diameters and order, [-]
          
          Examples
          --------
          >>> psd = PSDLognormal(s=0.5, d_characteristic=5E-6, order=3)
          >>> psd.fractions_discrete([1e-6, 1e-5, 1e-4, 1e-3])
          [0.0006434710129138987, 0.9165280099853876, 0.08282851796190027, 1.039798247504109e-09]
          '''
          cdfs = [self.cdf(d, n=n) for d in ds]
          return [cdfs[0]] + diff(cdfs)